Heat storage elements, a method for producing them and devices comprising heat storage elements

ABSTRACT

A heat-storing element comprising a storage substance which can absorb latent heat, seed crystals distributed uniformly in said storage substance and a structure which prevents any change of the distribution during the melting phase of the substance.

States 81311 1 91 Laing et al. 1M81'C111 13, 1973 [54] HEAT STORAGEELEMENTS, A [56] References Cited METHOD FOR PRODUCING TIIEM UNITEDSTATES PATENTS AND DEVICES COMPRISING I'IEAT STORAGE M NT 2,233,172lpvenson gig/:38 ournoy lnventorsl Nlkolflus a; Ingeborg Lama, 2,677,3675 1954 Telkes ..126/400 UX both of Hofener Weg 35-37, 7141 2,898,0918/1959 Verbeek... ..126/400 UX Aldingen bei Stuttgart, Germany 2,915,39712/1959 Telkes ..126/400 X 3,103,934 9/1963 Sabourin ..126/400 X FfledrJ ne 4, 1969 3,132,688 5/1964 Nowak ..126/400 x 3,463,161 8/1969Andrassy 126/400 X [21] Appl 830457 3,536,058 10/1970 Hearst ..126/400 x[30] Foreign Application Priority Data Primary ne ohn J Camby AssistantExaminer-W. C. Anderson June 6, 1968 21181113 ..A 5389/2: p i Edmonds,Morton. Taylor and June 6, 1968 ustria ..A 121 AdamS June 6, 1968Austria r ..A 1068/69 June 6, 1968 Austria ..A 1072/69 57 ABSTRACT June6, 1968 Austria ..A 5389/68 A heat-stormg element comprising a storagesubstance [52] U S Cl 126/400 which can absorb latent heat, seedcrystals distributed uniformly in said storage substance and a structure[51] Int. Cl. ..F24h 7/00 which prevents any change of the distributionduring [58] Field of Search ..126/400; 165/104, 105, 180,

the melting phase of the substance.

16 Claims, 6 Drawing Figures FIG. 1

PATENTEDMARmm ,720,19

SHEET. 20F 4 FIG. 3

PATENTEDMAR I 3l973 SHEET 30F 4 PATENTEDmmms 7 0,19

" sum ,u 0F 4 262. T v CD FIG. 50

HEAT STORAGE ELEMENTS, A METHOD FOR PRODUCING THEM AND DEVICESCOMPRISING HEAT STORAGE ELEMENTS THE PRIOR ART It is known-to utilizethe crystallization enthalpy of melts or solutions of crystallinematerials for heat storage purposes. Storage substances are also knownunder gravity in the melt either in the upward direction whichreversibly absorb latent energy on recrystalliza- 0 tion from one solidphase to another solid. The storage of laten heat utilizes the enthalpywhich can be calculated from the product of entropy and the absolutetemperature and which is equal to the difference of the internalenergies between two or more phases through which a materialsuccessively passes. The entropy is maximum after the vapor phase in theliquid state of aggregation. All polymorphously convertible crystallinesubstances can therefore absorb in their various conversion stages onlya part of the energy which can be absorbed when the storage substance isconverted to the liquid phase by a further supply of heat. Although theconversion of the solid to the molten state is desirablethermodynamically, its practical application gives rise to suchdifficulties that heat storage devices of this type have hitherto hardlybeen used. The disadvantages of solid-liquid conversion are due to thefact that the storage substances must be enclosed in containers toprevent the substances escaping. Such containersmust consist of amaterial which does not undergo any corrosion when the storagesubstances are in their liquid phase. Such containers are subjected toan extremely high mechanical load because on the transition from thecrystalline state to the molten state the storage substances undergoconsiderable variations in density. If the stored heat is to beexchanged .via a heat vehicle, e.g., a liquid or gas, heat exchangersare used which must be in satisfactory thermally conductive contact withthe storage substance. The heat exchangers must also have thinwalls,.and this also gives rise to insoluble technological problemsbecause of the extremely high forces occurring on the melting andsolidification ofthe storage substance. v

The economic problem is even greater, because in known storage-devices,for example of the kind used as cold accumulatorsin refrigerated trucks,the tanks cost about 100 times the cost of the s storage substance.

The main disadvantage-in the use of melting storage substancesaccommodated. in such tanks, however, is that the properties of thestorage substance vary over long periods. I

It is well-known that all melting crystals have a definite melting pointbut that solidification on removal of the heat takes place at atemperature which is often very much lower. Thermodynamically it isdesirable that the solidification temperature should coincide with themelting temperaturein order to avoid making the operating temperaturemuch higher than the required working temperature, since -.this wouldincrease the heat losses considerably during the storage time.

It is well-known that this effect is achieved by adding seed crystals tothe storage substance, the geometric (steric) structure of thesecrystals being so similar to the crystal shape of the storage substancethat during the cooling process they initiate crystallization near themelting temperature and thus prevent the cooling that would beinevitable without such seeding.

or, in most cases, downwards. The storage substance containing seedcrystals thus disintegrates increasingly so that the uniformdistribution of the crystals in the storage substance required forseeding to take place without delay is lost. For example, if the seedcrystals are deposited at the lowest point of the appliance, the heat isno longer yielded up at melting temperature, but at a lower temperature,so that the storage device cannot be used.

GENERAL DESCRIPTION OF THE INVENTION The invention relates to heatstorage elements whose thermodynamically active storage substance ismelted on heating and which, as a result of uniformly distributed seedcrystals, yields up the melting heat on cooling at approximately thesame temperature, known as the melting temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing therelationship between the reaction and the internal energy of a heatstorage substance undergoing a number of conversions between solid andmolten phases;

FIG. 2 is a diagrammatically prospective and partial section of astorage element constructed according to the invention;

FIG. 3 is a prospective view and section of a floor panel constructed inthe form of a hollow block, the interior of which contains a storageelement according to the invention;

- FIG. 4 illustrated a building panel or the like utilizing a plasticsheeting and enclosing a storage element according to the invention;

FIG. Sq illustrates a honeycomb form used as a secondary structure in astorage element according to the invention; and

FIG. 5b illustrates avfloor heating structure-utilizing the honeycombform of FIG. 5a.

DETAILED DESCRIPTION OF THE INVENTION To prevent the melt from becomingimpoverished in respect of seed crystals, the storage element accordingto the invention not only contains a storage substance and seed crystalsbut also a body which on the one hand retains the storage substance inthe liquid phase like a sponge or a foam, while on the other hand itfixes the seed crystals in their uniform distribution.

According to the invention, the rigidity of the body is so selected asto be shape-retaining, so that tanks can be fully dispensed with. Thuswith the invention it is possible to provide storage elements which,like the storage substance which makes use of polymorphy, are solids butwhich, unlike the polymorphous materials, enable all the enthalpy to beused for the storage process.

Storage substances have been described in which starch is added to thestorage substance to prevent leakage losses in the event of leakycontainers. This method is limited solely to a few hydrates and it hasbeen found that the resulting thixotropic material separates within afew months. It has also been proposed to impregnate balsa wood withstorage material in order to give shape retaining elements, but it hasbeen found that although balsa wood can be impregnated with liquidstorage substance, the seed crystals in solid form cannot penetrate thecell walls.

Suitable seed crystals are isotypical and epitaxial crystals providedthey are not dissolved in the storage substance melt and do notthemselves melt below the maximum operating temperature. Also suitableare isomorphous crystals if they are incorporated under the influence ofhigh surface energy, for example in gaps having an extremely smallopening angle. The capillary pressures are inversely proportional to thewall spacing. Increased pressures result in increased crystallizationtemperature so that penetration of the storage substance melt into finegaps increases the melting point. If this melting point increase isbeyond the maximum operating temperature, external crystals areunnecessary. According to the invention, therefore, there are added tothe storage substance, substances which have extremely fine gaps andwhich form a plurality of capillary passages to provide a high capillarytension with the storage substance melt. It has been found that groundglass fibers statistically have gaps in a number and configuration suchthat they result in a considerable increase in the melting temperatureof many storage substances. Such ground glass fibers can therefore beincorporated according to the invention in the body structure.

According to the invention, the preferred structure substances added arethose which as a result of elongate crystals can form structures similarto the zeolites. Zeolites are crystalline substances which can absorbconsiderable quantities of foreign substances, e.g., water, in theirvery loose crystal lattice structure. Since the amount of substancerequired for the structure is insignificant (as little as 2.5 percent byweight is frequently sufficient), the storage enthalpy values of storagesubstances according to the invention which are interspersedwith a bodyare hardly any different from those which are melted and recrystallizedin a container. If the weight of the container wall is also taken intoconsideration, the storage elements according to the invention are muchlighter. in particular, they are much cheaper and have none of the abovedisadvantages of the prior art storage devices, since in the liquidphase the synthetic zeolite as a solid holds the melt fast in itscrystal lattice structure. Thus it is impossible for a hydrostaticpressure to form anywhere; the crystal lattice is not affected by thechange of density as the material passes through the phases and finallystratification of the seed materials is impossible. If the storagesubstances in the molten state have a vapor pressure of the order ofmagnitude of water, evaporation must be prevented by a vapor barrier.According to the invention, this can be achieved by using plastics filmsor metal foils to enclose the body after the latter has been processedinto the form of mouldings. Alternatively, the surface can be sealed andrendered vaporimpermeable by chemical covering layers. Particularlysuitable layers of this type are those which consist of substances whichreact with a component in the air and thus pass over to awater-impermeable state only after application.

Particularly suitable substances for producing the body are those whichhave a fibrous crystal structure. These include fibrous or flocculentsilicates, such as aerosil, tripotassium or dipotassium silicate,calcium aluminates, the ferrites of the light metals, flocculent soot,magnesium oxide, silicon oxide and other flocculent crystals of otheroxides or salts.

It has been found advantageous according to the invention if the body isobtained by the use of crystal materials which are not soluble in thestorage substance melt at operating temperature but are partly solubleat elevated temperature. To ensure that the storage substance isuniformly incorporated, the invention generally proposes that the bodyforming substance should be mixed with the ground storage substance inthe solid state and be pressed after careful thorough mixing. Afterpressing, it has been found advantageous for heating to be carried outonce in order to release these crystals, so that the structur-formingcrystals interconnect in a sintering-like process. In the case ofhydrates, this heating can frequently be carried out only underpressure, since otherwise water of crystallization would vaporize out.Heating should be carried 7 out until a solid zeolite-like structure hasformed after 'recooling. It has been found advantageous to increase thetemperature until the solubility of the body-forming material is equalto about 30 percent. The amount of body-forming material depends on thenature of the storage substance and on its particle size in the solidstate. It has been found that generally very small quantities ofbody-forming substance are required. For example, 97 percent of aeutectic salt of a number of light-metal nitrates and 3 percentmagnesium oxide can be pressed to form storage substances which have aconsiderable strength even above the melting temperature of theeutectic. The seed crystals are also added to the material duringproduction. The quantity by weight is unimportant; only the number ofcrystals is significant. If the latter are very small, quantities of1-0.001 percent are sufficient. It is important that they shouldbe'uniformly distributed by mixing. After melting of the storagesubstance, the seed crystals remain uniformly distributed in the bodyand can no longer sink or rise. Given a suitable choice of the bodysubstance, the latter can also form seed material for the storagesubstance, so that there is no need to add any further seed substance.Although the reason has not been clarified, the formation of thezeolitic structure evidently results in the formation of body substancehydrates so that according to the invention a small amount of water canbe added to the storage substance during manufacture. Apart from thesaid metal oxides and double salts, according to the invention organicsubstances are also suitable for the production of bodies. A number ofhigh-polymer synthetic resins are advantageous. If required, they areadded with the addition of protective colloids to the hot melt. The meltis then emulsified at a temperature at which the substance for the bodyalready melts and the melt is solidified as abruptly as possible toprevent any separation.

A suitable coating material to prevent vaporizing, for both inorganicand organic bodies, is a mixture of 5 percent extremely finely groundbarite, 45 percent barium hydroxide and 50 percent ,water. The mouldingscan be dipped into this mixture. During the actual drying process bariumcarbonate forms at a uniform layer thickness and is water-insoluble andvapor-impermeable. Any other coating substances which do not react withthe storage substance and which pass over into water-insolublesubstances by atmospheric oxygen or carbon dioxide are also suitable.

All salts, oxides and hydroxides are suitable as storage substancesprovided they have a high melt enthalpy. Salts having a covalent orionic bond are therefore preferred. Very high enthalpy values areexhibited by hydrates, because of the hydrogen bridge bond, but theyoccur only in the region of relatively low temperatures. Since therequired storage temperatures rarely coincide with the natural meltingtemperatures, the invention proposes eutectic melts, and if ternary andquaternary eutectics are included,'a practically unlimited number ofstorage substances can be produced.

However, the invention is not confined to zeolite-like structure'systemsand organic foams, but can be embodied with other substances orstructures provided that seed crystals can be incorporated whichreliably prevent stratification and give a shape-retaining bodies. Theinvention therefore also uses element having a large residual volume,such as honeycomb shapes, glassfiber mats and open-pored hard foammaterials. The actual storage substance is mixed with a gel-forming orthixotropic-rendering substance in the storage substance liquid phaseand introduced into a plate, panel, sheet, or the like. The material ofthe latter takes all the tensile stresses and, where applicable, most ofthe pressure stresses, while the effect of the gel is to prevent thestorage substance from escaping inthe molten state. In this way it ispossible tomanufacture sheets, plates, panels, slabs or like elementshaving a very high mechanical strength. In the case of hard formmaterials or other solid elements, such as asbestos or felted paper, theseed crystal material is added to the body material during the actualmanufacture according to the invention in order to prevent their beingfiltered offon subsequent introduction. 7

If the storage elements are protected from vaporizing out or fromatmospheric humidity by films or foils, then according to the inventionthe latter can bemade selfrepairing in the event of any'damage by addingto the storage substance a chemical, such as barium hydroxideoctahydrate, for example, which converts to a water-insoluble substancein air.

In FIG. 1, the internal energy U of a storage substance for a storagematerial according to the invention is shown against the reaction (I).The level 1 corresponds to the internal energy of the solid. As far aslevel 2, this undergoes a polymorphous conversion, so that thedifference between the level- 2 and the level I can be taken as storageenergy. Level 3" corresponds to the molten state and shows that themaximum energy difference is between level 2 and level 3. It will beapparent that the inclusion of the molten. phase must in everycaseresult in a greater enthalpy than just the utilization of thepolymorphous conversions. This inclusion of the molten phase without theadverse stratification referred to hereinbefore, is the purpose of theinvention.

FIG. 2 diagrammatically illustrates a storage element 20 comprising astorage material 20' and a shape retaining body 20". The storagematerial as described previously may comprise a salt, for example ametallic salt, having high melt entholpy. The shape retaining body 20"may comprise generally a sponge, foam or a zeolite type structure whichis insoluable or substantially insoluable in the melted storage materialat its operating temperature and which has therein a plurality ofcapillary passages. Isotypical or epitaxial seed crystals, not shown, ofthe type previously described are incorporated in the shape retainingbody so that the seed crystals are in effect fixed in the storageelement and cannot over a period of time migrate in the storage materialwhen the storage material is in the molten phase.

The body member 20 being a sponge, foam or zeolite Structure willmaintain the molten storage material within the body due to capillaryforces. Where the storage material used has a vapor pressure on theorder of water, evaporation may be prevented by adding a vapor barrier,as for example film or sheet 24 which is also utilized to fix thestorage element to a wall 21 as at 23.

Floor heating systems have been constructed in the form of electricalstorage heaters which draw current at night and in which the heatstorage is carried out by ceramic materials. These heating systems areunsatisfactory, because the temperature rises proportionally with thestorage quantity, so that at times when a considerable amount of heat isrequired the morning temperatures are unacceptably high whereas theevening temperature 7 when there is the greatest heat requirement hasdropped greatly.

Instead of the storage elements for storing sensible heat, the inventionuses latent-heat-storage materials as described above. Storage materialsfor this purpose are required to have critical temperatures of about30C. To this end, according to the invention, use is made of thehydrates of non-metallic metal compounds, e.g., the decahydrate ofsodium sulphate or the dodecahydrate of disodium hydrogen phosphate. Thecontainer provided by the invention is preferably in the form ofthin-walled plastics tubes. According to the invention, they areaccommodated in elements which form the floor. Charging is viaresistance heaters which are disposed inside the storage material butpreferably between the tube filled with the storage material and theinsulation situated thereon; A gas-filled tube may be provided at asuitable place to exert a positive pressure on the storage material inthe gel state so that the storage material is pressed against theupwardly extending wall of the floor to ensure perfect thermal contact.Alternatively, according to the invention, the heat transfer can bereduced so that the heat yield is controlled by varying the pressure ina tube of this kind or else a hollow member is disposed between thefloorforming layer and the tube filled with the storage material, thegas filling of the tube being reducible until the layers bear completelyon one another. The element according to the invention can be used inthe same way as a wall or ceiling heating system.

FIG. 3 illustrates a floor panel or the like made of concrete and in theform ofa hollow block containing a plastic tube 50 having therein astorage element constructed according to the invention comprising astorage material 52 and a porous body element 52'. An air-filled tube 53is included to act as a pneumatic spring element. A heater 54constructed from two films, foils, sheets or the like having anelectrical resistance layer interposed therein is provided to charge thestorage material 52 during the night time. An insulating layer 55provides a thermal installation with respect to a level situated belowthe floor panel.

FIG. 4 illustrates a building panel utilizing heat storage elementsconstructed according to the invention where the elements are to besuspended from a wall. In this construction plastic sheets are joinedtogether to form cavities 250 which contain the heat storage elements.As the building panels would have little stiffness or rigidity, eyelets251 are provided in the structure 252 so that the panel may be hung froma wall. The plastic panels further serve as a vapor barrier to preventescape of vapor when the heat storage substance is in the molten phase.

FIG. 5a illustrates a fragmentary corner of a secondary body structurewhich may be includedin a heat storage element where the secondarystructure is in the form of a honeycomb extruded number. The edge walls261 of the element are reinforced with respect to the inner walls 262 sothat mechanically rigid panel elements are formed from whichcomplete-floor heating surface elements can be assembled.

' Referring to FIG. 5b there is illustrated a floor heater surfaceelement 263 which comprises individual elements 264 with floor plaster265v or the like thereabove over which a floor covering 266 is placed.Electrical heating elements 267 are disposed beneath the storageelements. As seen, each storage element 264 is constructed from atubular secondary structure 268 along with cover plates or foils 269which further serve to seal the storage material within the cells of thehoneycomb structure.

We claim:

1. A storage element comprising a storage substance which can absorbheat on transition from the solid state to the liquidstate having seedcrystals in said substance to prevent the storage substance from coolingbelow its melting temperature during a phase in which it yields up heat,characterized in that a porous shape retaining body having therein aplurality of small capillary passages is included in said element tohold said crystals uniformally throughout said substance and to holdsaidsubstance by capillary tension within said element anda vaporbarrier encasing said body to prevent escape of vapor of said storagesubstance from said element.

2. A storage element according to claim 1 characterized in that thestorage substance comprises substanaddition a plastic tube encasing saidstorage element.

6. A storage element according to claim 1 characterized in that saidvapor barrier comprises an air-tight flexible covering.

7. A storage element according to claim 1 characterized in that thestorage substance uniformly distributed in said body is incorporated ina further secondary body.

8. A storage element according to claim 1 characterized in that theporous body is made of ground glass fibers having fine gaps therebetweenwhich have a high surface tension in relation to the storage substancein the liquid state.

9. A storage element according to claim 1, characterized in that thebody comprises a porous material which, in the liquid phase of thestorage substance in the range of operating temperatures, is insolubleor substantially insoluble, does not react chemically with the storagesubstance and, at the melting temperature of the storage substances isin the solid state.

10. A storage element according to claim 9, characterized in that saidbody is at least slightly soluble in the storage substance melt at atemperature above operating temperature.

11. A storage element according to claim 9 characterized in that saidbody comprises a synthetically formed zeolite-like structures and thestorage substance is incorporated in said zeolite-like structures.

12. A storage element according to claim 1, characterized in that thestorage substance comprises a metallic salt hydrate which melts in itsown water of crystallization.

13. A storage element according to claim 12, characterized in that thestorage substance comprises a double salt hydrate.

14. A storage element according to claim 1, characterized in that thebody constitutes only a few per cent, preferably less than 3 percent, ofthe storage substance.

15. A storage element according to claim 1, characterized in that thesecondary body comprises a sheet metal mat.

16. A storage element according to claim 15, characterized in that thesecondary body comprises interlinked coils of metal whose diametercorresponds to the sheet tl'iickness.

1. A storage element comprising a storage substance which can absorbheat on transition from the solid state to the liquid state having seedcrystals in said substance to prevent the storage substance from coolingbelow its melting temperature during a phase in which it yields up heat,characterized in that a porous shape retaining body having therein aplurality of small capillary passages is included in said element tohold said crystals uniformally throughout said substance and to holdsaid substance by capillary tension within said element and a vaporbarrier encasing said body to prevent escape of vapor of said storagesubstance from said element.
 2. A storage element according to claim 1characterized in that the storage substance comprises substantialeutectic multi-phase mixtures.
 3. A storage element according to claim 1characterized in that the shape retaining body comprises an inorganicsubstance having fibrous crystal structures.
 4. A storage elementaccording to claim 1 characterized in that the storage material isuniformally distributed in said shape retaining body and has in additiona secondary support means integrated therein.
 5. A storage elementaccording to claim 1 having in addition a plastic tube encasing saidstorage element.
 6. A storage element according to claim 1 characterizedin that said vapor barrier comprises an air-tight flexible covering. 7.A storage element according to claim 1 characterized in that the storagesubstance uniformly distributed in said body is incorporated in afurther secondary body.
 8. A storage element according to claim 1characterized in that the porous body is made of ground glass fibershaving fine gaps therebetween which have a high surface tension inrelation to the storage substance in the liquid state.
 9. A storageelement according to claim 1, characterized in that the body comprises aporous material which, in the liquid phase of the storage substance inthe range of operating temperatures, is insoluble or substantiallyinsoluble, does not react chemically with the storage substance and, atthe melting temperature of the storage substances is in the solid state.10. A storage element according to claim 9, characterized in that saidbody is at least slightly soluble in the storage substance melt at atemperature above operating temperature.
 11. A storage element accordingto claim 9 characterized in that said body comprises a syntheticallyformed zeolite-like structures and the storage substance is incorporatedin said zeolite-like structures.
 12. A storage element according toclaim 1, characterized in that the storage substance comprises ametallic salt hydrate which melts in its own water of crystallization.13. A storage element according to claim 12, characterized in that thestorage substance comprises a double salt hydrate.
 14. A storage elementaccording to claim 1, characterized in that the body constitutes only afew per cent, preferably less than 3 percent, of the storage substance.15. A storage element according to claim 1, characterized in that thesecondary body comprises a sheet metal mat.